CB2: A Child Robot with Biomimetic Body for
Cognitive Developmental Robotics
Takashi Minato #1 , Yuichiro Yoshikawa #2 , Tomoyuki Noda ∗3 , Shuhei Ikemoto ∗4 ,Hiroshi Ishiguro #∗5 , and Minoru Asada #∗6#

Asada Project, ERATO, Japan Science and Technology Agency
Graduate School of Engineering, Osaka University
2–1 Yamada-oka, Suita, Osaka 565–0871, Japan
12

56

∗

{minato,yoshikawa}@jeap.org{ishiguro,asada}@ams.eng.osaka-u.ac.jp

Dept. of Adaptive Machine Systems, Graduate School of Engineering, Osaka University
2–1 Yamada-oka, Suita, Osaka 565–0871, Japan34{tomoyuki.noda,shuhei.ikemoto}@ams.eng.osaka-u.ac.jp

Abstract— This paper presents a new research platform, CB2 ,a child robot with biomimetic body for cognitive developmental
robotics [1] developed by the Socially-Synergistic Intelligence(Hereafter, Socio-SI) group of JST ERATO Asada Project. The
Socio-SI group has focused on the design principles of communicative and intelligent machines and human social development
through building a humanoid robot that has physical and perceptual structures close to us, that enables safe and close interactions
with humans. For this purpose, CB2 was designed, especially inorder to establish and maintain a long-term social interaction
between human and robot. The most significant features of CB2
are a whole-body soft skin (silicon surface with many tactile
sensors underneath) and flexible joints (51 pneumatic actuators).
The fundamental capabilities and the preliminary experiments
are shown, and the future work is discussed.

I. I NTRODUCTION
This paper presents a new research platform, “CB2 ”, aChild-robot with Biomimetic Body for cognitive developmental robotics [1] developed by the Socially-Synergistic
Intelligence (Hereafter, Socio-SI) group of JST ERATO Asada
Project. Based on the cognitive developmental robotics, theAsada project aims at understanding the human developmental process of cognitive functions through mutual feedback
between scientific and synthetic approaches by building ahumanoid robot in which a computational model of thedevelopment is embedded. Among four different groups [2],the Socio-SI group has focused on the design principles ofcommunicative and intelligent machines and human social
development through building a humanoid robot that has
physical and perceptual structures close to us, that enables
safe and close interactions with humans. For this purpose, CB2
was designed, especially in order to establish and maintain along-term social interaction between human and robot.
We focus on a development of timing control and temporal concepts which may be required for a development ofcommunication skill. Here the timing control basically means
to create a temporal condition which is most effective for a
response [14]. To enable to control action timing toward others

Fig. 1.

The developed humanoid robot CB2 .

and surrounding environments is a necessary developmental
process for robots or infants to establish a communication withothers. The sense of time is not associated with a specificsensory organ unlike the vision or audition; it is inducedby other perceptions. In other words, the sense of time isstrongly relevant to other different perceptions. It is, therefore,possible to discover not only a factor to induce a function
development but also interactions between developments ofvarious functions through studying the development of timing
control. We perform the preliminary studies on the developmental mechanism of timing control in terms of sensory-motororganization, interpersonal responsive behaviors, and motion
capabilities in order to construct developmental models of both

TABLE IC OMPARISON OF HARDWARE SPECIFICATION .
Joint flexibility
Robovie-II [3]Infanoid [4]BARTHOC [5]iCub [6]Robovie-IIF [7]ASIMO [8]QRIO [9]SAYA [10]Repliee R1 [11]Repliee Q2 [12]Geminoid [13]CB2No(Electrical motors)
No(Electrical motors)
No(Electrical motors)
No(Electrical motors)
No(Electrical motors)
Yes(Pneumatic actuators)
No(Electrical motors)
Yes(Pneumatic actuators)
Yes(Pneumatic actuators)

Actuators mounted
throughout whole-bodyNo(Upper body)No(Upper body)

HumanlikeappearanceChild size

No

No

No

Yes

No

Yes

No

Yes

Yes

No
(Upper body)

No

No

No

Yes

No

Yes

No
(No tactile sensor)
No(No tactile sensor)

No(Head)
No(Head)
No(Upper body)

Yes

No

Yes

Yes

Yes

No

Yes

Yes

Soft and sensitive skin
No
No

Yes
Yes

human and robot.
The ability to communicate and establish relationships withhumans cannot simply be programmed into a robot. The robot
must develop these skills much as humans do from earlychildhood on. Moreover, just as in childhood, other humans’
help is necessary to aid normal development. To this end,
it is necessary to develop a robot which is able to elicit
others’ help and to interact safely and tightly with persons
in many ways, such as ”help the robot stand up by holding it”
and ”teach a body motion by directly moving its limbs.” Inother words, it is necessary that assistive behaviors of human
helper naturally appear during the developmental process of
the robot’s behaviors. We therefore have designed and built
a new humanoid robot CB2 that is able to tightly and safely
interact with persons in daily life in advance of constructing
the developmental models. CB2 has a humanlike appearance
similar to a child-sized boy. It also has much more actuators
throughout its body to make human-like motions than theexisting human-size humanoid robots. To make the interaction
more natural and safer, we use flexible pneumatic actuators,
and the body is covered in soft silicone skin. Various sensory
organs are also necessary. CB2 has two cameras inside its
eyes, two microphones in the ears, and tactile sensors beneaththe silicone skin. Table I compares CB2 with the typicalhumanoid robots for studying human-robot communicationand human development with respect to the presence of several
features which are required for natural and tight human-robot
interaction. The joint flexibility and the soft and sensitive skinprovide tight interaction with humans. The humanlike motion
owing to the actuators mounted throughout whole-body andthe childlike appearance invite child-directed behaviors fromhumans. Only CB2 provides all the features and therefore it
can be said that CB2 is a novel research platform for studying
a developmental mechanism through human-robot interaction.
The next section shows the specifications of CB2 , and three
preliminary but essential studies using CB2 are introduced

Yes

Parallel
link
neck

eyes

} Motordrivenleft arm

Parallel
link

waist

right arm

left handeye brows

left legcheeksmouth

right handright leg

Parallellink=
Parallellink
Fig. 2.

Active joint driven
by air actuator= Air cylinder

The kinematic structure of CB2 .

with their future works.
II. T HE DEVELOPMENT OF CB2 AS A NEW RESEARCH
PLATFORM
2

CB (Fig.1) is a new research platform for studying adevelopmental mechanism through human-robot interaction inthat CB2 has all of the following features.
• It has a humanlike appearance similar to a child-sizedboy. It is about 130cm high and weights about 33kg.
• It has fifty-six actuators (Fig.2).

Fig. 3. An example of flexible whole-body movement: an infant’s general
movement.

It has flexibly controlled joints thanks to pneumatic
actuators.
• The whole-body is covered in soft silicone skin (Fig.5).
• It has many tactile sensors throughout its body (Fig.6).
As shown in the kinematic structure of Fig.2, CB2 has totally
fifty-six actuators; the eyeballs and eyelids are driven byelectrical motors since quick movements are required for theseparts while the other body parts are driven by pneumatic actuators. The joint driven by the pneumatic actuator has mechanical flexibility in the control thanks to the high compressibilityof air. The pneumatic actuators mounted throughout the wholebody enables CB2 to generate flexible whole-body movements.
Although the mechanism is different from a human, it cangenerate humanlike behavior (although it has a more limited
range of movements than that of humans). Fig.3 shows anexample of baby-like motion in the similitude of an infant’s
general movement. Taga et al. have shown that the chaotic
dynamics of the neural system is a source of variability of
the general movements [15]. In order to generate chaotic
movements, the joint angle trajectories in the lower half of the
body were generated from a trajectory on the Lorenz attractor
in this example. CB2 can generate not only a fidgety motion
but also a dynamic whole-body motion. Fig.4 shows such an
example motion. CB2 turns over onto its left side by making
use of the inertia forces of the legs. It is difficult to stand up,
walk, and run by itself because the power of these actuators is
not sufficient. These motions are however possible, if a person

Fig. 4.

An example of dynamic whole-body movement: rolling over.

•helps it. In addition to gross body movements, CB2 can also
make facial expressions although these are not as rich as thoseof a person. Existing robots typically use powerful electrical
motors, which make it dangerous for us to tightly interact
with them. The soft skin and flexible actuators (which do notstrongly fight against an external force) increase the safety andpromote feelings of security, inviting tighter interaction frompeople. Moreover, we can expect that the childish appearance
and humanlike movement elicit interpersonal behaviors toward
CB22 from people.
The pneumatic actuator is controlled by an air flow control valve (the operating pressure is constant (0.7 MPa)).
Although the output torque of the pneumatic actuator cannot
be controlled, the response characteristic varies owing to theflow rate. The joint driven by the pneumatic actuator can bepassively moved by releasing the compressed air. CB2 has also
an artificial vocal tract (rather than a simple speaker) and can
produce a voice quite similar to human vowel sounds using
the flexible vocalization mechanism [16].
The vision is realized by two cameras mounted inside theeyeballs, and the audition is realized by two microphones
mounted both on the head, and located externally, around CB2 .
The tactile sensation is realized by the tactile sensors using
PVDF films embedded beneath the skin. The sensor output



Fig. 5.

(b) The skin structure.
&KOGPUKQPThe tactile sensor.

TactilesensorFig. 6.

&KOGPUKQP

Urethane form(a) The silicone skin and tactile sensors.

Silicone skin
PVDF

The tactile sensor layout.

is proportional to the rate of change of bending (deformationrate). The information equivalent to a contact force is obtainedby temporal integration of the sensor output. The tactile sensors are put between urethane foam covering the mechanicalparts and the silicone skin. A touch on the skin where no
sensor is located can be detected because the deformation ofurethane foam is spatially spread. 197 tactile sensors cover thewhole body as shown in Fig.6 and realize a whole-body tactile
sensation. All sensor outputs can be read in 100 Hz.
CB2 is not a stand-alone system: the control valves, aircompressor, and computers for controlling actuators and sensor information processing are placed outside the body. In spiteof this, it can continuously work for a long period (for all theday).
III. P RELIMINARY STUDY WITH CB2 – SELF - ORGANIZEDSENSORY- MOTOR INTEGRATIONAs robots become more ubiquitous in our daily lives, thephysical distances between humans and robots decreases. This
increases the potential for robots to inadvertently harm users.
Moreover, it becomes increasingly important for robots to beable to interpret the meaning of touch (or haptic) feedback
from humans. Covering the whole robot body with malleabletactile sensors addresses both of these concerns. First, softFig. 7. The somatosensory map of CB2 . In this map the sensors which havea high correlation with each other are closely placed. The map is generated across-correlation between 197 sensor outputs obtained when a person touches
CB2 ’s hand, head, torso and so on.

and compliant surfaces are less dangerous in the event ofaccidental human impact. Second, flexible sensors are capableof distinguishing many different types of touch (e.g., hard v.s.
gentle stroking). The soft skin has the additional advantage ofinviting more natural types of touch interaction from humans.
Unfortunately, the processing information from these typesof sensors presents a difficult challenge. To get a better
understanding of this domain, we have constructed a databaseof recorded interactions between humans and a robot equippedwith tactile sensors covering the whole body (Robovie-IIF [7]),and performed several classification and visualization tasks.
Using a novel feature space based on cross-correlation between
tactile sensors, we have found that interaction scenarios inwhich we can expect subject’s touch could be successfullyclassified simple k-nearest neighbors, achieving performanceof 60% in a 13-way forced choice task [17]. We have also
found that many categories of touch interactions can be easily
visualized by arranging sensors into a “Somatosensory Map”
using multidimensional scaling (MDS) applied to this feature
space as a similarity measure. On the other hand, the maps
visualized from categories without touch interactions showeda same distribution of sensors. This means that the cross correlations in a short time window are stable except during touch
interaction. Fig.7 shows the “Somatosensory Map” derived
from tactile sensor signals obtained when CB2 is touchedat the head, the hands, and the body during a interaction
with a person. This map is different from the self-organizedsomatosensory map shown in [18] in that a structure of theactual task (haptic interactions with a person) is reflected in themap. The interactions are also visualized in the map showing
distinctive clusters consisting of the touched parts of sensors.
These promising results suggest that this feature space can beeffectively used for automatic analysis of touch behaviors inmore complex tasks and in various types of robots.
While Robovie-IIF has built-up constructed skins separated
at the joints, CB2 has a seamless skin in terms that the materials embedding tactile sensors are not separated at the multiple
joints. This feature always causes various self-touches whichare tactile senses caused by own movements, and requires self
movement information for classifying tactile sensor signals. If
it is possible to assume that the robot knows a tactile sense
caused only by self movement in advance, it can be removed
from the total tactile senses. However, this assumption is notsatisfied because a part of the robot’s body is always in contact
with something in the real world (e.g., robot’s feet are always
in contact with a floor). Furthermore, a problem to discriminatea factor caused by self movement from other actors can be seenin the human development process. This kind of problem is
supposed to be solved by introducing a temporal concept. In
the future, we explore a mechanism to discriminate self fromothers through developing a method to associate a tactile sensewith a body movement based on the classification method wehave developed.
IV. P RELIMINARY STUDY WITH CB2 – DEVELOPMENT OF
MULTIMODAL COMMUNICATION

In addition to the childlike properties, CB2 is provided
with multimodal sensing capabilities, such as vision, audition,and touch, to capture what humans usually perceive. They
are supposed to be a basis of the CB2 ’s potential to exhibit
responding behaviors in a way how a person usually anticipates in other persons’ responding behaviors. Furthermore, it isthereby expected to derive human’s interpersonal perceptions
and behaviors in the interaction with CB2 , which are originally
supposed to be shown in human-human interaction.
Such responsiveness is regarded as an important elementwith which a person gives others an impression that theperson is a communicative being for the others [19]. To reveal
feasible mechanisms for such responsiveness, we have started
an experiment with an interactive anthropomorphic on-screen
agent by focusing on the effects of the response timing. In theexperimental setup, a participant was asked to sit across froman on-screen agent that was designed to blink in response to
the participant’s blinking, and then answer on a question, ”didshe feel that the on-screen agent looked at her?”. Yoshikawa
et al. reported that the agent could give participants stronger
feeling of being looked at by it if it responded with not
too rapid but not too slow latencies [20]. This result implies
that controlling the timing of response even with a subtle
channel of communication such as blinking could change theparticipants’ impression on an interactive agent. In the future,
we expect that we can model how a person anticipates inother’s responses through revealing the relationship between
multimodal channels and the timing of them in the experimentswith CB2 .
Furthermore, it is a formidable issue to answer on a developmental question, how a robot or infant can acquire such
response timing of multimodal channels and the mechanism
to recognize others based on such timing. Since persons
might unconsciously anticipate CB2 to respond in a childlike
way due to its childlike properties, it could be a promisingexperimental tool to model on which ways of responding
persons forms impression of child-likeness and exhibits childdirected behaviors for CB2 . Through such experiments, we

also plan to approach to the mysteries on the infant/robot
cognitive development of ’others’ concepts from the viewpointof what kinds of robot’s responses can derive a caregiver’s
child-directed behaviors and how these behaviors can help or
guide the development.
V. P RELIMINARY STUDY WITH CB2 – SOCIALLY MOTORDEVELOPMENT

The flexible joint driven by the pneumatic actuator provides
high safety and robustness against breakdown caused by
an external force. This fact promotes that people physicallyinteract with a robot movement and enables physical humanrobot interaction with a simple control method. This can beadvantage against existing robots driven by powerful electrical
motors. We then focus on the physical interaction between aperson and CB2 owing to its flexibility. As the example, wehave implemented that a person helps CB2 up by holding its
arms (hereafter, raising up interaction). This section describes
the implementation and studies on motor development throughphysical human-robot interaction.
In order to implement the raising up interaction, CB2 needs
to actively and passively change its posture in accordance withan external force applied by a person. We have implemented
a method to control the angles of all joints driven by thepneumatic actuators by gain scheduling control. If the control
gain is small, CB2 passively changes the posture in accordancewith the applied force. On the other hand, CB2 keeps the joint
angles in the desired positions against the applied force if thegain is large. The applied force can be detected by measuring
the angular error on the actively driven joint.
The raising up interaction is implemented in the following
procedure.
1) Define the CB2 ’s initial posture.
2) Define the final desired posture and gain to keep astanding position without an external disturbance.
3) Define the intermediate desired posture.
4) Turn the timing to switch the desired posture.
The resultant interaction is shown in Fig.8. The desired
postures, gain, and switching timing were experimentally
determined.
In the figure 8, the postures 1, 2, and 3 denote the initial,
intermediate, and final desired postures, respectively. The robot
switches the desired position from position 2 to 3 during
the interaction. Although the desired posture discontinuously
changed, the actual joint angles smoothly changed thanks to
the joint flexibility. The result shows that the flexibility not
only contributes to safety and robustness but also facilitates
the design of control system.
The raising up interaction shown in Fig.8 can be interpreted
as a kind of communication between the human helper androbot in which the physical forces are transmitted to eachother. We think of the physical instruction as a physical communication which can be quantitatively analyzed and study the
motor development of the robot with a person’s physical help.
During the motor development with physical help, the robot

Posture 1

Posture 2
Fig. 8.

Posture 3

CB2 is standing up with a person’s help.

(learner) is supposed to acquire how to minimize the supporting effort given by the person (teacher) while learning thedesired motion. Meanwhile, a development of communication
ability involves a maximization of information gained fromother’s physical expression (e.g., language and gesture), that
is, a minimization of the other’s physical expression to gain
information. In other words, a learner acquires how to interpretthe other’s intention as much as possible from the physicalexpression. From the analogy between two developments,
a process to minimize the teacher’s effort corresponds theprocess of communication development. We then study amechanism to promote a social motor development in the
viewpoint that decreasing a teacher’s supporting force in the
motion learning is relevant to a development of ability tointerpret other’s intention, that is, to communicate with others.
Viewed in the light that the timing tuning was important in
the implementation of the raising up interaction, we especially
focus on a relation between the motor development and adevelopment of timing control.
VI. C ONCLUSION
This paper reported the development of a new humanoid
robot CB2 that has a soft skin and flexible actuators anddescribed studies on developmental mechanisms thanks tothe characteristic of CB2 . These studies associate with eachother in elucidation of the mechanism of human development.
CB2 enables long-term and tight interaction with people, and,
therefore, it can be a research platform to study a developmental mechanism in which various factors are combined in acomplicated way. It contributes not only to a development ofcommunicative and intelligent robots but also to understanding
the development of human intelligence.
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